As the only natural satellite of the Earth, the Moon is the brightest object in the night sky. Considerably smaller than Earth, the Moon is only about 27 percent the radius of Earth. Its mass is also much smaller, resulting in a relatively weak lunar gravity. The Moon's orbit is moving away from Earth at a rate of 1.5 inches (3.8cm) a year. The Moon is scarred by impact craters and has remained largely unchanged for billions of years. Today the Moon remains the only celestial body visited by man. NASA Apollo astronauts landed on the Moon six times, between 1969 and 1972.

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